1. Field of Endeavor
The present invention relates to optical materials and more particularly to a crystal for electro-optic applications.
2. State of Technology
Background information is provided by U.S. Pat. No. 5,945,037 for an optical polarizer material patented to Christopher A. Ebbers, Aug. 31, 1999, which includes the following information, “An electromagnetic wave (such as light from a laser) is characterized by its direction of propagation, frequency, amplitude and polarization. The polarization corresponds to the direction parallel to the plane (and normal to the propagation direction) in which the amplitude of the wave rises and falls. A polarizer acts to alter this direction, either by absorption or reflection of light waves with the incorrect polarization. Light from an incoherent source such as a light bulb consists of many super-imposed electromagnetic waves with random, relative polarizations. Passing this light through a polarizer allows only those light waves with the desired polarization to pass through. An example of a polarizer of the first type (absorption) is Polaroid film found in many sunglasses. Polarizers of the second type (reflection) are formed using thin film dielectric coatings, parallel wire grids (used for wavelengths typically > than 1 micrometer) and birefringent crystals (predominantly calcite (CaCO.sub.3)). Polarization altering components such as waveplates are almost exclusively made from quartz (SiO.sub.2). Lasers which are high peak power (large energy per pulse/pulse width) or high average power (large energy per pulse multiplied by the pulses per second) risk damaging the Polaroid or wire grid polarizers. Lasers such as these must use the thin film polarizers or calcite polarizers. Calcite polarizers are the “best” polarizers for a number of applications. The extinction ratio of a polarizer is a measure of how well that polarizer operates at to produce a specific polarization state of light. By placing together two similar polarizers with their polarization directions at right angles to each other, theoretically no light should be transmitted through those two polarizers. The ratio of the intensity of the measured transmitted light to the intensity of the incident light is known as the extinction ratio. Polaroid polarizers typically have an extinction ratio of 1:10,000. Thin film polarizers have typical extinction ratios of 1:1000. Calcite polarizers have the highest extinction ratios in the range of 1:100,000 to 1:1,000,000. Thin film polarizers are typically manufactured for a single wavelength, and thus are wavelength sensitive. Calcite polarizers have a higher extinction ratio, have a damage resistance as high as that of thin film polarizers, and are broadband. They are usable in the range of 2000 nm to 250 unm. Calcite is a naturally occurring mineral mined from the earth. The chemical formula is CaCO.sub.3. The best calcite is mined in northern Mexico, where it was deposited by naturally occurring geothermal processes. However, most of the calcite mined is unsuitable for optical use, due to veils, inclusions, and other crystalline defects. Also, although the theoretical transmission of calcite extends roughly from 200 nm to 4000 nm, the practical absorption in the far infrared and near ultraviolet (in the mined crystal) is limited by the ionic impurities (such as Fe) which were present in the water in which the calcite grew. These problems inherent in mined calcite would be reduced by synthetic growth methods. Unfortunately, if calcite is directly heated at atmospheric pressure, it decomposes to CaO and CO.sub.2 before it melts. (This is true of most carbonates.) To grow calcite, it is necessary to duplicate the high pressure and temperature found in the earth. This entails growing calcite by a hydrothermal method. While hydrothermal methods are used extensively to grow quartz in industrial quantities, only experimental quantities of synthetically grown calcite are available. Industrial hydrothermal growth methods are much more expensive than low temperature melt growth methods, and calcite can not be grown by the standard low temperature melt growth methods. In addition, calcite grown using industrial hydrothermal methods contains microinclusions of water, degrading its performance in the infrared wavelength regime (due to optical absorption by the water inclusions) as well as in the ultraviolet spectral regime (due to scattering by the submicron water inclusions. Calcite remains, after more than a century, the principal material for Nicol prisms in the polarizing microscope. As natural stocks are depleted the need for growing synthetic CaCO.sub.3 in large 1-2″ minimum size boules becomes more urgent. Thus, a need exists for a replacement material for calcite as a polarization material.”
Background information is provided by U.S. Pat. No. 6,185,231, patented Feb. 6, 2001, and U.S. Pat. No. 6,327,282, patented Dec. 4, 2001, for a Yb-doped:YCOB laser to Dennis A. Hammons which include the following information, “A tunable, solid state laser device with both visible and infrared laser emission is developed with a trivalent ytterbium-doped yttrium calcium oxyborate crystal as the host crystal. The Yb:YCOB crystal generates an infrared fundamental light over a wide bandwidth, from approximately 980 nanometers (nm) to approximately 1100 nm. The bandwidth generated by the Yb:YCOB crystal is approximately 100 nm wide and supports the generation of pulsed infrared light or when self-frequency doubled provides a compact, efficient, source of tunable, visible, blue or green laser light in wavelengths of approximately 490 nm to approximately 550 nm.”
Background information is provided by U.S. Pat. No. 6,330,097 to Qiushui Chen, et al. for a high-speed electro-optic modulator, patented Dec. 11, 2001 which includes the following information, “Much progress has been made in the last thirty years in developing optical switches or modulators, but current devices are not very satisfactory for many applications. The majority of active fiberoptic devices used in present day systems, for example, fiberoptic intensity attenuators, are based on electromechanical operation. In one type, fibers are positioned end to end and mechanically moved in or out of line. In another type, mirrors are rotated to direct beams into or away from a receiving fiber. This can be accomplished mechanically or with piezoelectric or electrostatic drivers. Mechanical devices intrinsically lack speed and long term reliability. Solid-state light controlling devices (without moving parts) are needed for fiber communication systems. A key problem for these developing fiberoptic components is realizing speed and reliability, as well as the essential fiberoptic systems requirement of low insertion loss and polarization insensitivity. For devices used between regular fibers, low insertion loss and polarization insensitivity operation is the basic performance requirement. Others have proposed an optical switch/attenuator using a liquid crystal cell as the modulation element situated between an input and an output birefringent element, each fed by optical fibers. When the liquid crystal cell is turned on, light emerging from the output birefringent element is deflected and not focused by the subsequent collimator onto the corresponding optical fiber. Although it has the desirable features of low insertion loss, and low required operating voltage, being liquid crystal-based, the long term reliability of organic materials and the relatively low switching speed are not suitable for many applications. Others have also proposed a fast (less than one microsecond) optical switch using an electro-optic crystal in which birefringence can be induced by application of an electric field. Operation is based on rotating the plane of polarization of light with respect to the orientation of a subsequent passive polarizer that blocks or transmits light depending on the angle. The basic arrangement works efficiently with incoming light polarized with a particular orientation. Randomly polarized light suffers a loss. This is overcome by using additional elements that split incoming light into two orthogonal polarizations, passively rotates one to match the other, and combines the two into a single beam fed to the basic modulator. However, the suggested electro-optic crystals, require voltages of a kV or more for operation. Still others have described a modulator having a tapered plate, a Faraday rotator or electro-optic crystal, and a second tapered plate. The Faraday rotator is controlled by varying the current in an external coil which varies a magnetic field. The suggested electro-optic crystals require high drive voltages of kilovolts. Electrode design also effects polarization dependence and modulation efficiency.”
Background information is provided by U.S. Pat. No. 6,335,816 for Pockels cell and optical switch with pockels cell to Jacques Luce, patented Jan. 1, 2002 which includes the following information, “By Pockels cell is meant an electro-optical cell able to change the polarisation of a beam crossing through it via the application of an electric field to the cell. Said cell may be cut in birefractive crystal whose cristallographic axes are deviated by the presence of an electric field parallel to the optical axis of the crystal. This warrants the name ‘Pockels cell’ with longitudinal field. Regenerative amplifiers use a trigger switch in two states. In a first state, photons are trapped and amplified in a laser cavity, while in the second state the photons are removed from the cavity. To achieve the switch function, systems are used with which the polarisation of the laser beam can be switched by 90° by means of return excursion in a Pockels cell controlled by an electric voltage. Conventionally a KDP (potassium-dihydrogen-phosphate) crystal bar is used having a length of 2 to 3 cm, whose ends are provided with electrodes. To carry out optical switching in two states, it is necessary to successively apply two independent voltages of high potential (for example 4000 V) to each electrode in order to set up or cancel a polarising electric field in the cell. The quick changeover from one voltage to the other requires costly and complex electronic switching equipment. It proves to be difficult to provide electronic switching equipment which achieves both fast set-up of a high voltage on a terminal and fast cancellation of this voltage. To avoid this difficulty, the single Pockels cell switch is replaced by a switch with two Pockels cells.”